I have found video (russian language).
They are using Sigil software. At first, they open epub and on the left they have epub structure (epub is simply zipped xhtml document with exact structure). They are editing style*.css file (it contains paths to the fonts) and then add their own fonts to the Fonts folder. After that they save new epub with embedded fonts and check in ADE (Adobe Digital Editions) if it is working. I use different lines in style*.css, but it seems their definitions work also.
Link is here